1. Field of the Invention
This invention relates to a process for injection molding ultra high molecular weight polyethylene.
2. Description of the Prior Art
In general, ultra high molecular weight polyethylene has excellent impact strength, abrasion resistance, self lubrication properties, and chemical resistance and is becoming more widely used for many purposes.
However, its melt viscosity is very high and its fluidity is poor compared with other widely used resins. Therefore, it is very difficult to shape ultra high molecular weight polyethylene by extrusion or injection molding. As a result it has conventionally been shaped by compression molding, the working efficiency of compression molding is so low that the art has long sought an injection molding method with a short molding cycle and high efficiency.
When ultra high molecular weight polyethylene is shaped by using conventional injection molding techniques, melt fracture occurs during filling the polyethylene into the mold cavity and the product is formed into easily peelable, mica like layers. Additionally, the molten polyethylenes, which are joined in the mold, often are not completely fused and this causes block breaking. Polyethylene injected through a conventional nozzle does not form continuous strands as with the usual resins. Instead it forms powders by flashing as it is charged in the mold with the result that often deaeration is not satisfactorily completed. Therefore, the thus molded product fails to show the excellent properties inherent in ultra high molecular weight polyethylene, and is inferior in such mechanical properties as tensile strength, impact strength, abrasion resistance and the like compared with conventional polyethylene products.
Various methods have been proposed for obtaining good products from resins of a high melt viscosity by injection molding, but when such methods are applied to ultra high molecular weight polyethylene the results are not always satisfactory. For example, the above mentioned drawbacks such as layer-like peeling, poor fusion and insufficient deaeration are improved, to some extent, by the process of heating the mold to a high temperature upon injection and cooling after injection. However, the improvements are not sufficient to permit the production of quality products. Additionally, the method requires a long molding cycle and is, therefore, unsatisfactory from an economical point of view. It has also been proposed to reduce the cavity volume of the mold after injection of the high polymer. For example, Japanese Patent Publication No. 1664/1965 and Japanese Patent Laid-Open-to-Public No. 43047/1972, describe such process. But the procedures are not effective for the production of high quality ultra high molecular weight polyethylene products.
When the polymer is injected into a mold cavity of a volume slightly larger than a volume of the final molded article at a shear rate obtained with conventional injection nozzles used for injection molding of polymers, and the cavity volume is then reduced to the selected volume, the resulting molded article is mica like in appearance and easily separable into layers.
Porous molded articles of ultra high molecular weight polyethylene can be manufactured by sintering a powder material, but the working efficiency is so low that the method is not suitable for mass production.